Fig. 4

Vertical subluxation. (a) Lateral radiograph with McGregor’s line (black line; 61-year-old man with RA). The tip of the dens is difficult to define, but measurement according to Redlund-Johnell’s method (white line) results in a distance of 27 mm, which is below the normal limit. In accordance with this, the anterior arc of the atlas is level with the middle third of the axis. (b) Ranawat’s method, the distance between the centre of the second cervical pedicle and the transverse axis of the atlas is below the normal limit (9 mm). Thus, all measurements indicate vertical subluxation. Supplementary MRI, (c) sagittal STIR and (d) T1-weighted images show erosion of the dens and protrusion of the tip into the occipital foramen causing narrowing of the spinal canal to 9 mm, but persistence of cerebrospinal fluid around the cord. There is a 9-mm-thick mass of pannus tissue between the dens and anterior arc (black line) exhibiting small areas with high signal intensity on the STIR image (arrow) compatible with slight activity, but signal void fibrous pannus tissue predominates